you only need one combobox having the currencies.
Then you need an array, or db query to find a matching value, and the factor for it.
If youre base currency is €, and your target currency is $, then you only need the number to be multiplied from $ to €.
You can read these once from a file, and then access them from an array.
I would write a XML file, for the ease of reading, and changing it.
Youre XML could be looking like this:
<currency label="USD" symbol="$" factor="0,95" />
<currency label="EUR" symbol="€" factor="1" />
<currency lable="YEN" symbol="Y" factor="unknown" />
then you can read all labels into the combo, and if the usere selected one, you can directly read the factor an present the symbol